The present invention relates to analog integrated circuits.
The invention concerns an integrated circuit adapted to supply other integrated circuits with a reference voltage (or reference current) which is stable in value. This is particularly advantageous with telecommunications circuits. The following description will make reference to this field of application, for convenience of illustration, but it should be understood that the claimed inventions are not necessarily limited to this field of use.
Integrated circuits for telecommunications applications (such as Subscriber Line Interface Circuits) are quite complex. (Discussion of Subscriber Line Interface Circuits may be found in U.S. Pat. Nos. 4,800,589 to Siligoni et al., 4,897,872 to Siligoni et al., and 5,046,089 to Pariani et al.; all of which are hereby incorporated by reference.) In order to perform correctly in accordance with their design specifications, many such integrated circuits need to be supplied a reference current I.sub.ref which is stable over time. (This reference current is critical, since several circuit parameters depend on it.) To provide this current, the telecommunications circuit is usually associated with an integrated circuit voltage regulator which outputs a stable reference voltage, from which the reference current is derived. The disclosed innovations provide an improved voltage regulator, and improved system, of this type.
Conventional voltage regulator circuits incorporate several resistors, and this fact unavoidably poses some difficulties in integrated circuit implementations. Normal integrated circuit fabrication processes produce wide unpredictable variations in resistor values, in the resistances of doped semiconductor regions, and in the characteristics of active devices. Typically the circuit designer must allow for tolerances of .+-.20% in the designed resistor values, and for even wider variations in transistor gain. (Laser trimming or other special process steps can be used to adjust the value of the as-fabricated resistors, but such steps are expensive.) Moreover, the resistances of doped semiconductor regions, and the gain of transistors, may vary strongly with temperature. (See generally S.Sze, PHYSICS OF SEMICONDUCTOR DEVICES (2.ed. 1981); A. Grove, PHYSICS AND TECHNOLOGY OF SEMICONDUCTOR DEVICES (1967); VLSI TECHNOLOGY (2.ed. Sze 1988); S.Sze, SEMICONDUCTOR DEVICES, PHYSICS AND TECHNOLOGY (1985); A. Glaser & G. Subak-Sharpe, INTEGRATED CIRCUIT ENGINEERING (1977); A. Milnes, SEMICONDUCTOR DEVICES AND INTEGRATED ELECTRONICS (1980); B. Streetman, SOLID STATE ELECTRONIC DEVICES (3rd ed. 1990); and R. Muller & T. Kamins, DEVICE ELECTRONICS FOR INTEGRATED CIRCUITS (1986); all of which are hereby incorporated by reference.)
Much work has been expended on developing circuits for providing a stable reference voltage. One particularly important family of circuits is those referred to "bandgap voltage reference" circuits. Such circuits generally use a structure wherein the difference between base-to-emitter voltage drops at two different emitter current densities appears across a resistor. Since this differential voltage exhibits variation opposite to that of other components (e.g. a forward-biased junction diode), it provides a tool which can be used to achieve a regulated voltage which is reasonably independent of temperature and supply voltage.
A variety of circuits have been proposed for voltage reference circuits. A pioneering publication was Widlar, "New Developments in IC Voltage Regulators," 6 IEEE JOURNAL OF SOLID-STATE CIRCUITS 2ff (1971), which is hereby incorporated by reference. Other important developments are described in Brokaw, "A Simple Three-Terminal IC Bandgap Reference," 9 IEEE JOURNAL OF SOLID-STATE CIRCUITS 388ff (1974), which is hereby incorporated by reference. Expository discussions of this area of design may be found in P. Gray & R. Meyer, ANALYSIS AND DESIGN OF ANALOG INTEGRATED CIRCUITS (2.ed. 1984) (which is hereby incorporated by reference in its entirety), especially at pages 275-296 thereof; in D. Feucht, HANDBOOK OF ANALOG CIRCUIT DESIGN (1990) (which is hereby incorporated by reference in its entirety), especially at pages 522-547 thereof; and in J. Scott, ANALOG ELECTRONIC DESIGN (1991) (which is hereby incorporated by reference in its entirety), especially at pages 69-88 thereof. Other background on voltage and current reference circuits includes the following items, all of which are hereby incorporated by reference: U.S. Pat. Nos. 5,125,112, 5,119,015, 5,103,159, 5,087,830, 5,084,665, 5,081,410, 4,785,231, 4,714,872, 4,651,083, 4,647,841, 4,628,248, 4,596,948, 4,528,495, 4,498,041, 4,412,347, 4,361,797, 4,308,496, 4,297,646, 4,251,743, 4,059,793, 4,055,774, and 3,922,596; 1989 ISSCC DIGEST OF TECHNICAL PAPERS at 120-121; EDN vol. 33, no. 2, pp. 147-54; IEEE JOURNAL OF SOLID-STATE CIRCUITS vol. SC-22, at pp. 71ff (February 1987); Hart et al., "The Design of Constant Current Sources," ELECTRONIC ENGINEERING pp. 85-88, vol. 49, No. 593, (June 1977); and Caveliere et al., "Integrated transistor voltage/temperature regulator," IBM TECHNICAL DISCLOSURE BULLETIN vol. 25, no. 9 p. 4863 (February 1983).
However, as will be discussed in detail below, the existing circuits still exhibit large power consumption, and/or second-order sensitivity to variations in as-fabricated resistor values, which are undesirable for integrated circuits.